The invention relates to a hydraulic piston made from rolled sheet metal, to a method of manufacturing such a piston and to a blank forming an intermediate product of said method. Pistons of this type are used e.g. in hydraulically actuated vehicle brakes.
Pistons of the described type have to satisfy mutually conflicting requirements: on the one hand, they are to be light so as to have a low inertia, on the other hand, they have to be stable enough to prevent radial deformation of the side wall of the piston in the event of the base of the piston being subjected to a high load, such as occurs e.g. during a powerful braking operation because of the high hydraulic pressure in the brake system. If the last condition is not met, a total loss of function of the piston may occur.
Hydraulic pistons usually have the shape of a cylindrical cup with an open end. In a portion of the outer surface of the side wall adjoining the open end of the piston there is often an annular groove, which is used to fasten a protective sleeve. The region of the annular groove is particularly at risk of load-induced material deformation because the wall is weakened by the groove.
From WO 91/12445, and corresponding U.S. Pat. No. 5,231,916, both of which are incorporated by reference herein, a piston manufactured by a drawing method is known, in which the wall thickness in the critical groove region is not smaller than the wall thickness in the other regions of the piston. Two critical regions are mentioned in WO 91/12445, namely, on the one hand, the region of the groove and, on the other hand, the inner region of a spring arrangement. The stability requirements in these two regions determine the thickness of the piston wall.
From the Russian patent 2 163 987 a drawing method for piston manufacture is known, which enables a piston to be manufactured with a stable, yet thin wall. The piston is a hollow structure with an open end. Its outer wall is cylindrical in shape and has a groove for fastening a protective sleeve. The inner surface of the piston is cylindrical in a region adjoining the open end and extending in axial direction internally over the annular groove. In a further inward lying portion of the piston the inside diameter is greater than in the region adjoining the open end. Consequently, the thickness of the side wall is greater at the open end than in the vicinity of the piston head, this being beneficial in terms of the ability to withstand axial forces in the critical region of the groove. It is however complicated to manufacture such a piston having two inner surfaces of differing inside diameter and identical outside diameter, especially as the inner surface with the smaller inside diameter lies at the open end of the piston.